Predicting Products/Balancing Equations Online Help
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Practice Problem #5
Here are the key steps (in order) to doing stoichiometry:
1. Identify the type of reaction [combustion, synthesis, decomposition, single-replacement, or double replacement].
2. Write the products of the reaction based on reaction type.
- combustion: always produces CO2 + H2O
- synthesis: combines the elements, then write the chemical formulas canceling out the charges
- decomposition: breaks up the compounds into elements, being aware of the diatomic elements
- single-replacement: cations take the place of cations, and anions take the place of anions; then write the chemical formulas canceling out the charges
· double-replacement: cations take the place of cations, and anions take the place of anions; then write the chemical formulas canceling out the charges
3. Then, balance the equation.
Sample Example Problem [to show you how it’s done]:
Problem: AlCl3 (aq) + Pb(NO3)2 (aq) –> ?
1. Identify the type of reaction.
Double-replacement reaction.
You have 2 compounds made up of cations and anions.
AlCl3 (aq) + Pb(NO3)2 (aq) –>
2. Write out the products of the chemical reaction.
Switch the cations, since this is double-replacement:
AlCl3 + Pb(NO3)2 –> Pb Cl + Al NO3
Then write the formulas of the products based on their charges.
Pb = +2, Cl = -1, so the formula is PbCl2.
Al = +3, NO3 = -1, so the formula is Al(NO3)3.
AlCl3 + Pb(NO3)2 –> PbCl2 + Al(NO3)3
3. Balance the equation.
AlCl3 + Pb(NO3)2 –> PbCl2 + Al(NO3)3
The number of Al atoms is equal on both sides (both have 1).
Next, there are 3 Cl atoms on the left, 2 on the right.
Lowest common multiple is 6. See coefficients below.
Both multiply the Cl atoms to 6.
2AlCl3 + Pb(NO3)2 –> 3PbCl2 + Al(NO3)3
Next, the are 2 Al atoms on the left, 1 on the right.
Lowest common multiple is 2. See coefficients below.
Both multiply the Al atoms to total 2 atoms Al on either side.
2AlCl3 + Pb(NO3)2 –> 3PbCl2 + 2Al(NO3)3
Next, there is 1 Pb atom on the left, 3 on the right (because of 3x1).
2AlCl3 + 3Pb(NO3)2 –> 3PbCl2 + 2Al(NO3)3
Last, check the nitrates (NO3), which now both have 3 on either side.
This is the same as checking the nitrogens and the oxygens by themselves.
Answer to Problem: 2AlCl3 + 3Pb(NO3)2 –> 3PbCl2 + 2Al(NO3)3
Note: Printing this out might help.
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